Methods and apparatus for tuning an LC Filter

ABSTRACT

A receiver, such as a television receiver, tunes an inductive (“L”) and capacitive (“C”) filter based on a desired frequency. The LC filter includes a plurality of inductors, configured in at least one inductive (“L”) bank, and a plurality of capacitors configured in at least one capacitive (“C”) bank. The inductors are selectively enabled for the LC filter by an N code, and the capacitors are selectively enabled for the LC filter by an M code. The receiver uses a functional comparator to compare offset, capacitive, inductive and frequency values to tune the LC filter. Techniques for tuning the LC filter for operation in the VHF and UHF bands are disclosed.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional PatentApplication No. 60/386,644, filed Jun. 5, 2002, entitled “FrequencyDiscrete L-C Tank for TV Tuner”, and U.S. Provisional Patent ApplicationNo. 60/386,471, filed Jun. 5, 2002, entitled “Functional Comparator forBinary L-C bank Addressing.”

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention is directed toward the field of discretepassive filters, and more particularly toward a tunable LC filter bank.

[0004] 2. Art Background

[0005] Typically, receivers employ filters to condition both inputsignals and internally generated reference signals. For example,bandpass, notch, and low pass are types of filters employed inreceivers. The frequency response of a filter refers to thecharacteristics of the filter that condition the signal input to thefilter. For example, a bandpass filter may attenuate an input signalacross a pre-determined band of frequencies above and below a centerfrequency of the filter. Filters are designed to exhibit frequencyresponses based on one or more circuit parameters.

[0006] Some receivers are designed to process input signals with a rangeof input carrier frequencies (e.g., wide band receivers). For example,television receivers must be capable of processing input televisionsignals with carrier frequencies ranging from 55 MHz to 880 MHz. Onecircuit parameter used to define the frequency response of a filter isthe carrier frequency of an input signal. Thus, such wide band receiversrequire filters to generate multiple frequency responses to accommodatemultiple input carrier frequencies. To accomplish this, some receiversemploy tunable filters to process a wide band of input frequencies.

[0007] One type of tunable filter is a varactor type tuner. A popularapplication for the varactor is in electronic tuning circuits, such astelevision tuners. A direct current (“DC”) control voltage varies thecapacitance of the varactor, re-tuning a resonant circuit (i.e.,filter). Specifically, a varactor diode uses a pn junction in reversebias such that the capacitance of the diode varies with the reversevoltage. However, the relationship between the control voltage and thecapacitance in a varactor tuner is not linear. Thus, the capacitancevalue is based on the signal level. This non-linearity producesdistortion in the output of the filter (e.g., a third order product).

[0008] Other receivers, such as television receivers, may employ activefilters. The use of a continuous or active filter requires a powersupply voltage (e.g., V_(cc)). The power supply voltage exhibits aripple due to noise on the voltage supply line. This ripple voltage, inturn, causes unacceptable frequency response characteristics on theoutput of the continuous amplifier. Accordingly, it is desirable to usediscrete or passive filters in the receiver to isolate the signal fromripple voltage, thereby improving signal quality.

SUMMARY OF THE INVENTION

[0009] A receiver, such as a television receiver, tunes an inductive(“L”) and capacitive (“C”) filter based on a desired frequency. The LCfilter includes a plurality of inductors, configured in at least oneinductive (“L”) bank, and a plurality of capacitors configured in atleast one capacitive (“C”) bank. The inductors are selectively enabledfor the LC filter by an N code, and the capacitors are selectivelyenabled for the LC filter by an M code.

[0010] In order to tune the receiver for a channel frequency in the VHFspectrum, the receiver determines a first offset between an initialresonant frequency of the LC filter and the channel frequency (i.e., thedesired frequency for which the LC filter is tuned). For course tuning,the N code is determined based on an initial value for the M code, thedesired frequency, and the first offset. At least one inductor in the Lbank is selected based on the N code. Then, a second offset between aresonant frequency of the LC filter and the desired frequency isdetermined. For fine-tuning, the M code is determined based on the valuefor the N code, the desired frequency, and the second offset. At leastone capacitor in the C bank is selected using the M code.

[0011] In order to tune the receiver for a channel frequency in the UHFspectrum, the receiver also determines a first offset between an initialresonant frequency of the LC filter and the desired frequency. Forcourse tuning, the M code is determined based on an initial value forthe N code, the desired frequency, and the first offset. At least onecapacitor in the C bank is selected from the M code. A second offsetbetween a resonant frequency of the LC filter and a desired frequency isdetermined. To fine-tune the LC filter, the N code is determined basedon the value for the M code, the desired frequency, and the secondoffset. At least one inductor in the L bank is selected from the N code.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a block diagram illustrating one embodiment for atelevision tuner that utilizes LC bank filters.

[0013]FIG. 2 is a block diagram illustrating one embodiment forimplementing the LC bank filters in a television tuner.

[0014]FIG. 3a illustrates one embodiment for an inductive (L) bank foruse in the LC filter bank.

[0015]FIG. 3b illustrates one embodiment for a capacitive bank for usein the LC filter bank of the present invention.

[0016]FIGS. 4a and 4 b are flow diagrams illustrating one embodiment fortuning the LC filter bank for a channel in the VHF spectrum.

[0017]FIG. 5 shows one embodiment for selecting inductors in an inductorbank from the N code.

[0018]FIG. 6 illustrates various resistances for selected inductances ofthe L bank.

[0019]FIG. 7 is a graph that depicts the relationship between the centerfrequency of an LC bank filter and the total capacitance as a functionof the M code.

[0020]FIG. 8 depicts relationships between the selected M code andcenter frequency for various combinations of the N code.

[0021]FIG. 9 shows the information for capacitance and M code forselecting capacitors in a C Bank during VHF tuning.

[0022]FIG. 10 shows various resistances for selected capacitances of theC bank.

[0023]FIGS. 11a and 11 b are flow diagrams illustrating one embodimentfor tuning the LC filter bank for a channel in the UHF spectrum.

[0024]FIG. 12 shows one embodiment for selecting capacitors in acapacitor bank for UHF tuning.

[0025]FIG. 13 is a graph that depicts the relationship between thecenter frequency of an LC bank filter and the total inductance as afunction of the N−1 code.

[0026]FIG. 14 depicts relationships between the selected N-1 code andcenter frequency for various combinations of the M code.

[0027]FIGS. 15a and b show the information for selecting inductors in anL Bank during UHF tuning.

[0028]FIG. 16 illustrates various resistances for selected capacitancesof the C bank.

[0029]FIG. 17 is a timing diagram that shows timing for tuning the LCfilter bank in accordance with one embodiment.

[0030]FIG. 18 illustrates one embodiment for a functional comparatorcircuit used in one embodiment for tuning the LC filter bank.

[0031]FIG. 19 illustrates one embodiment for a calculator used in thefunctional comparator circuit of FIG. 18.

[0032]FIG. 20 illustrates a plurality of frequency responses for oneembodiment of the LC filter bank.

DETAILED DESCRIPTION

[0033] The disclosures of U.S. Provisional Patent Application No.60/386,644, filed Jun. 5, 2002, entitled “Frequency Discrete L-C Tankfor TV Tuner”, and U.S. Provisional Patent Application No. 60/386,471,filed Jun. 5, 2002, entitled “Functional Comparator for Binary L-C bankAddressing” are hereby expressly incorporated herein by reference.

[0034] The present invention includes one or more inductive (“L”) andcapacitive (“C”) filter banks applied to realize a non-varactor typetelevision tuner. In one embodiment, the television tuner is integratedinto a single integrated circuit chip. The LC banks are used toimplement passive filters. The television tuner optimally selectscombinations of inductors and capacitors in the LC bank to tune thetelevision receiver.

[0035]FIG. 1 is a block diagram illustrating one embodiment for atelevision tuner that utilizes LC bank filters. The television tuner 100receives a radio frequency (“RF”) television signal, and generatesdemodulated baseband television signals (i.e., picture and soundsignals). For this embodiment, television tuner 100 includes inductivebanks 110 and 124, as well as capacitive banks 115 and 126. Inductivebank 110 and capacitive bank 115 comprise LC filter bank 112. Similarly,inductive bank 124 and capacitive bank 126 constitute LC filter bank125. As described fully below, LC filter banks 110 and 115 provide aband pass filter function for television receiver 2100.

[0036] The television circuit 100 also includes inductors 102 and 104 tofilter the input RF signal. For this embodiment, the inductors 102 and104 values are set to 21.8 nano henries (“nH”) and 91.2 nH,respectively. An automatic gain control circuit 120 amplifies thesignal, output from LC filter bank 112, for input to the second LCfilter bank 125. Inductor 122, with a value of 91.2 nH, adds a parallelinductance to LC filter bank 125. As described more fully below, LCfilter banks 112 and 125 generate a band pass frequency response forconditioning of the input signal.

[0037] The television tuner 100 contains one or more down conversionstages. For this embodiment, television tuner 100 includes two quadraticdownconverters. A first quadratic downconverter is implemented usingmixers 130 and 132, local oscillator 140, and notch filter 150. Thefirst quadratic downconverter converts the frequency of the filtered RFtelevision signal to a first immediate frequency (e.g., 45.75 mega hertz(“MHz”)). In general, a quadratic demodulator splits the input signal,and mixes the input signal with an in-phase (“I”) local oscillatorsignal and a quadrature phase (“Q”) local oscillator signal. The Q localoscillator signal is phase shifted 90 degrees from the I localoscillator signal.

[0038] A second quadratic downconverter circuit, which receives theoutput signal from the first quadratic downconverter circuit, includesmixers 160 and 165, local oscillator 170, notch filter 180, and bandpass filter 190. The second quadratic downconverter converts thefrequency of the first intermediate television signal to a secondimmediate frequency (e.g., 10.5 mega hertz (“MHz”)).

[0039] The television receiver also includes IF processing 195. The IFprocessing module 195 generates baseband picture and sound carriercomponents. One embodiment for IF processing that utilizes a quadraticdemodulator to process a television signal is described in United Statespatent application Serial Number [EXPRESS MAIL NO.: EL 909174649],entitled Quadratic Nyquist Slope Filter, filed Sep. 30, 2002, which isexpressly incorporated herein by reference.

[0040]FIG. 2 is a block diagram illustrating one embodiment forimplementing the LC bank filters in a television tuner. A televisionreceiver 200 includes inductive “L” banks A and B. For this embodiment,the L banks are implemented external to an integrated circuit 200. The Lbank “A” consists of five inductors (202, 204, 206, 208, and 209).Similarly, L bank “B” contains five inductors (212, 214, 216, 218, and219). Each inductor of inductive bank A is electrically coupled tointegrated circuit 200 through an input/output (“I/O”) pad (i.e., pad A1couples conductive 202, pad A2 couples inductors 204, pad A3 couplesinductive 206, pad A4 couples inductor 208, and pad A5 couples inductor209). Similarly, an I/O pad is provided for each inductor in L bank B(i.e., pad B1 couples inductor 212, pad B2 couples inductor 214, pad B3couples inductor 216, pad B4 couples inductor 218, and pad B5 couplesinductor 219). A switch is provided for each inductor in both L banks Aand B (switches 203, 205, 207, 208 and 211 for L bank A, and switches213, 215, 217, 221 and 223 for L bank B). A total inductance isgenerated for each L bank by selectively coupling the external inductorsto the television receiver circuit through the switches. Specifically, adigital code (hereafter referred to as the N code) is applied to theswitches to select inductors in the L bank. In one embodiment, theswitches are implemented using metal oxide semiconductor (“MOS”)transistors.

[0041] L bank A has a corresponding capacitive (“C”) bank A, labeled 220in FIG. 2. Similarly, L bank B has a corresponding capacitive bank B,labeled 210 in FIG. 2. In one embodiment, the C bank contains fourcapacitors per bank. As shown in FIG. 2, C bank 220 includes a pluralityof capacitors (225, 222, 224, and 227) coupled to a plurality ofswitches (232, 231, 229 and 228). The capacitors are selected using acode (hereafter referred to as an M code), to control the switches thatcouple the capacitors to the television receiver circuit.

[0042] The television receiver circuit 200 includes circuitry to selector program the LC filter banks. In general, television receiver 200generates the M code and N code to selectively program the LC filterbanks. By selecting different combinations of inductors (L bank) andcapacitors (C bank), different filter characteristics (i.e., frequencyresponses) are realized. For the embodiment shown in FIG. 2, televisionreceiver 200 receives information, referred to as a channel code, thatspecifies the desired television channel. In one embodiment, the channelcode is received by bus transceiver 270, using its external pins (ENB,SCL, and SDA). A programmable divider 240 is used to digitize at least aportion of the channel code (e.g., 10 bits). The programmable divider240 is also used to digitize an LC oscillation frequency. The LCoscillation frequency is the center frequency for a tuned cavitygenerated by the combination of selected inductors and capacitors fromthe LC banks. The LC oscillation frequency is generated using amplifier(AGC) 230. The tuning circuit for television receiver 200 includes atiming reference, and associated circuitry to generate various timingsignals. An oscillator circuit, which uses a 16 MHz crystal (255), iscoupled to a divider 250 to generate timing signals.

[0043] Television receiver 200 also includes a plurality of digital toanalog (D/A) circuits 262 to convert digital values to analog currents.In one embodiment, the analog currents are used in a calculator 264 anda comparator circuit 268 for tuning of the LC filter banks. Register 272stores a digital value, A. The digital values for M code and N code arestored in registers 274 and 276, respectively. As shown in FIG. 2,digital values (A, M, N and F_(LC), and frequency of the channel code“F_(ch)”) are input to D/A circuits 262. As shown in FIG. 2, thecalculator 264 and comparator circuit 268 generates a value for A, the Mcode and the N code using the timing from timing circuit 260.

[0044]FIG. 3a illustrates one embodiment for an inductive (L) bank foruse in the LC filter bank. For this embodiment, the inductive bankincludes five inductors (315, 320, 325, 330 and 340). Although theinductive bank 300 includes five inductors, any number of inductors maybe used without deviating from the spirit or scope of the invention. Inone embodiment, the number and values for the inductors is a function ofthe desired frequency response characteristics of the LC filter bank.The inductors, which form inductive bank 300, are configured inparallel. For the embodiment of FIG. 3a, the inductor values are 5.7,11.4, 22.8, 45.6, and 91.2 nH. Each inductor is added to the L bankthrough a corresponding switch (switches 310, 308, 306, 304 and 302). Inone embodiment, the switches are implemented using metal oxidesemiconductor (“MOS”) transistors.

[0045]FIG. 3b illustrates one embodiment for a capacitive bank for usein the LC filter bank of the present invention. For this embodiment,capacitive bank 350 contains five capacitors (360, 362, 364, 366 and368). For this embodiment, the capacitor values are 3.7, 9.4, 17, 32.8and 64.6 pF. A different number of capacitors and different capacitivevalues may be selected to implement filters for the LC filter bank withdifferent frequency responses. Also, as shown in FIG. 3b, capacitors360, 362, 364 and 366 are selected for the C bank through switches 358,356, 354 and 352, respectively. In one embodiment, the switches areimplemented with MOS transistors.

[0046]FIGS. 4a and 4 b are flow diagrams illustrating one embodiment fortuning the LC filter bank for a channel in the VHF spectrum. The processis initiated by selecting an initial value for the inductance, L, (i.e.,N code) and capacitance, C (i.e., M code) (block 400, FIG. 4a). In oneembodiment, M is set to a value of binary 0100, and N is set to a valueof binary 0001. A variable, A, is used to determine an offset betweenthe currently tuned LC oscillator frequency, F_(LC), and the desiredtuned channel frequency, F_(ch). In one embodiment, A comprises a fivebit digital value. The variable, A, is set to an initial condition(block 410, FIG. 4). In one embodiment, the initial condition is equalto “10000.”

[0047] The circuit obtains a sample value for the LC oscillatorfrequency (“F_(LC)”) (block 420, FIG. 4a). The LC oscillator frequencyis converted from an analog signal to a digital value using a high-speeddivider. The comparator circuit (285, FIG. 2) is used to evaluate theexpression:

CF _(LC) ² =A ²(1/L)

[0048] In one embodiment, digital values for C (i.e., M code), F_(LC),A, and L (i.e., N code) are converted to analog currents using a digitalto analog converter, and the analog currents are input to a calculator,to generate both numeric sides of the expression, and a comparator toevaluate the expression (See FIGS. 11 & 12). If the expression is nottrue (i.e., CF_(LC) ² does not equal A²(1/L)), and CF_(LC) ² is greaterthan A²(1/L), then the digital value for A is incremented (blocks 425,427 and 430, FIG. 4a). For example, after the first evaluation, ifCF_(LC) ² is greater than A²(1/L), then the initial value of A, 10000,is incremented to the value of 10001. If the expression is not true(i.e., CF_(LC) ² does not equal A²(1/L)), and CF_(LC) ² is less thanA²(1/L), then the digital value for A is decremented (blocks 425, 427and 428, FIG. 4a). For example, after the first evaluation, if CF_(LC) ²is less than A²(1/L), then the initial value of A, 10000, is decrementedto the value of 01111. This process repeats until the expressionevaluates to true. When the expression evaluates to true, the offset, A,has been calculated. The offset is stored in a register for subsequentuse (block 440, FIG. 4a).

[0049] To tune the input VHF channel, a value for the N code is firstdetermined. The selection of N (inductive selection) results in coarsetuning a channel in the VHF spectrum. The circuit sets an initial valuefor the N code (block 450, FIG. 4a). In one embodiment, N is set to avalue of 00001. Also, values are selected for M and F_(ch) (block 455,FIG. 4a). In one embodiment, the M code is set to a value of 0100, andF_(ch) is set to the value of the channel code (i.e., the desired tunedfrequency).

[0050] A circuit evaluates the expression:

CF _(ch) ² =A ²(1/L).

[0051] In one embodiment, digital values for C (i.e., M code), F_(ch),A, and L (i.e., N code) are converted for evaluation of the expression.If the expression is not true (i.e., CF_(ch) ² does not equal A²(1/L)),then the digital value for N is incremented (blocks 465 and 470, FIG.4a). For example, after the first evaluation, if CF_(LC) ² is not equalto A²(1/L), then the initial value of N, 00001, is incremented to thevalue of 00010. This process repeats until the expression evaluates totrue. When the expression evaluates to true, the N code has beendetermined, and the L bank is set based on the N code (block 455, FIG.4a).

[0052] The process re-calculates the offset, A, after the course tuningprocedure. The variable, A, is set to an initial condition (e.g., 10000)(block 462, FIG. 4b). The circuit obtains a new sample value for the LCoscillator frequency, F_(LC), after the course tuning (block 464, FIG.4b). The frequency, F_(LC), is converted from an analog signal to adigital value using a high-speed divider. The following expression isevaluated:

CF _(LC) ² =A ²(1/L)

[0053] If the expression is not true (i.e., CF_(LC) ² does not equalA²(1/L)), and CF_(LC) ² is greater than A²(1/L), then the digital valuefor A is incremented (blocks 466, 468 and 470, FIG. 4b). If theexpression is not true (i.e., CF_(LC) ² does not equal A²(1/L)), andCF_(LC) ² is less than A²(1/L), then the digital value for A isdecremented (blocks 466, 468 and 469, FIG. 4b). This process repeatsuntil the expression evaluates to true. When the expression evaluates totrue, the new offset, A, has been calculated. The offset is stored in aregister for subsequent use (block 472, FIG. 4b).

[0054] Next, a value for the M code, and consequently C, is determined.The selection of M results in fine tuning a channel in the VHF spectrum.First, an initial value is set for M (block 474, FIG. 4b). In oneembodiment, M is initially set to a value of 0000. Also, the values forF_(ch) and N code are selected. F_(ch) represents the frequency of thechannel code. The N code was set from the coarse tuning stage. Theexpression, F_(ch) ^(1.5)C=A^(1.5)(1/L), is evaluated to determine if itis true (block 478, FIG. 4b). In one embodiment, these digital valuesare converted to analog current values using digital to analogconverters. The analog current values are then weighted in accordancewith the expression (e.g., A^(1.5), F_(ch) ^(1.5), etc.) using acalculator, and then compared using a comparator (See FIGS. 11 and 12).If the expression does not evaluate to true, the M code is incremented(block 480, FIG. 4b), and the expression is again evaluated with the newM code. This process is repeated until the expression evaluates to true.With the expression evaluates to true, the capacitive bank is set basedon the M code (block 490, FIG. 4b).

[0055]FIG. 5 shows one embodiment for selecting inductors in an inductorbank from the N code. The first column of FIG. 5 shows inductor valuesfor the corresponding inductors in the inductor bank shown in column 2.For example, inductor L5 has a value of 5.7 nH, while inductor L1 has avalue of 91.2 nH. Note that inductor L0 is always selected. A binary Ncode for selecting inductors in the inductor bank is shown. For example,column 3 shows selection of inductor L0 for the N code of 00000. Thelast column of FIG. 5 shows selection of inductors from the inductorbank for the N code of 00111. Specifically, the “00111” N code specifiesselection of inductors L0, L1, L2 and L3 from the inductor bank. Thelast row of FIG. 5 shows the total inductance for the corresponding Ncode. For example, for the “00000” N code, the total inductance of theinductance bank is equal to 91.2 nH. The total inductance for the“00111” N code is equal to 11.4 nH.

[0056] The inductors and capacitors are selected from the C and L banks,respectively, the use of switches (e.g., MOS transistors). A resistanceis introduced into the LC bank filter response by each transistor. Thus,each capacitor selected in the C bank increases the series resistance.The increase in series resistance, or decrease in parallel resistance,decreases the Q factor, which, in turn, degrades performance of the LCbank filter.

[0057] In general, a Q factor is measured based on the expression:

Q=2¶fRC

[0058] In one embodiment, the receiver selects a combination ofinductors and capacitors to configure an LC filter bank so as tomaximize the Q factor. As shown by the above expression, the larger theparallel resistance and capacitance, the greater the Q factor. It is anobjective to maximize the Q factor through proper selection ofinductance and capacitance combinations from the LC banks.

[0059]FIG. 6 illustrates various resistances for selected inductances ofthe L bank. Specifically, column two lists a resistance, r_(sl), fromthe corresponding inductor. For example, L5 has a resistance of 0.7ohms. The third column lists the resistance, R_(MOS), from the MOStransistor for the corresponding inductor. For example, L4 has a R_(MOS)resistance of 1.6 ohms. The fourth column lists the total seriesresistance, r_(s), from selecting the corresponding inductor. Forexample, L3 has a total series resistance, r_(s), of 2.8 ohms. The lastrow of FIG. 6 lists the total series resistance for the corresponding Ncode. For example, an N code of 00010 has a total series resistance of2.4 ohms. The circuit selects combinations of the inductors from an Lbank to reduce the total series resistance and thus maximize the Qfactor.

[0060]FIG. 7 is a graph that depicts the relationship between the centerfrequency of an LC bank filter and the total capacitance as a functionof the M code. As shown in FIG. 7, for center frequencies in the VHFspectrum, a higher M code (i.e., overall capacitance) yields a higherfrequency. FIG. 7 also depicts the relationship between the M code andcenter frequency of the LC bank filter (and the desired channel) foreach value of the N code.

[0061] In order to maximize the Q factor, only certain combinations ofthe N code and M code are used. FIG. 8 depicts relationships between theselected M code and center frequency for various combinations of the Ncode. Note that the relationship between the center LC bank filterfrequency and the M code is now confined to the center of a graph. Thisrelationship minimizes the Q factor, and optimizes the response of theLC filter bank.

[0062] As discussed above, the tuning circuit of the present inventiononly selects certain combinations of N and M codes to configure the LCbank filter. FIG. 9 shows the information for capacitance and M code forselecting capacitors in a C Bank during VHF tuning. The first column ofFIG. 9 lists the total capacitance, which ranges from 3.7 pF to 127.7pF, for the C bank. The second column identifies an M code to obtain thecorresponding capacitance value in column one. For example, a decimal Mcode of 12 yields a total capacitance of 101.3 pF. Column 3 of FIG. 9shows valid selections for the M code based on the frequency and thevalue of the N code. For example, in column 3, a list of frequencies isshown for valid selections of the M code when the N code is equal to 1.Specifically, column 3 shows that for these frequencies and N code, theM code has a valid range between 8 and 16 (decimal).

[0063]FIG. 10 shows various resistances for selected capacitances of theC bank. Column two lists a resistance, R_(MOS), for a correspondingcapacitor. For example, M code 10 (decimal) has a resistance of 1.8ohms. The circuit selects combinations of the capacitors from a C bankto reduce the total series resistance and thus maximize the Q factor.

[0064]FIGS. 11a and 11 b are flow diagrams illustrating one embodimentfor tuning the LC filter bank for a channel in the UHF spectrum. Theprocess is initiated by selecting an initial value for the inductance,L, (i.e., N code) and capacitance, C (i.e., M code) (block 700, FIG.11a). In one embodiment, M is set to a value of binary 0001, and N isset to a value of binary 10000. The variable, A, is set to an initialcondition (block 710, FIG. 11a). In one embodiment, the initialcondition is equal to “10000.”

[0065] The circuit obtains a sample value for the LC oscillatorfrequency (“F_(LC)”) (block 720, FIG. 11a). The LC oscillator frequencyis converted from an analog signal to a digital value using a high-speeddivider. The following expression is evaluated:

CF _(LC) ² =A ²(1/L).

[0066] If the expression is not true (i.e., CF_(LC) ² does not equalA²(1/L)), and CF_(LC) ² is greater than A²(1/L), then the digital valuefor A is incremented (blocks 725, 727 and 730, FIG. 11a). For example,after the first evaluation, if CF_(LC) ² is greater than A²(1/L), thenthe initial value of A, 10000, is incremented to the value of 10001. Ifthe expression is not true (i.e., CF_(LC) ² does not equal A²(1/L)), andCF_(LC) ² is less than A²(1/L), then the digital value for A isdecremented (blocks 725, 727 and 728, FIG. 11a). For example, after thefirst evaluation, if CF_(LC) ² is less than A²(1/L), then the initialvalue of A, 10000, is decremented to the value of 01111. This processrepeats until the expression evaluates to true. When the expressionevaluates to true, the offset, A, has been calculated. The offset isstored in a register for subsequent use (block 740, FIG. 11a).

[0067] To tune the input UHF channel, a value for the M code is firstdetermined. The selection of M (capacitive selection) results in coarsetuning a channel in the UHF spectrum. The circuit sets an initial valuefor the M code (block 750, FIG. 11a). In one embodiment, M is set to avalue of 00001. Also, values are selected for N and F_(ch) (block 755,FIG. 11a). The N code is set to a value of 10000, and F_(ch) is set tothe value of the channel code (i.e., the desired tuned frequency).

[0068] A circuit evaluates the expression:

CF _(Ch) ^(1.5) =2 (A ^(1.5)(1/L)).

[0069] If the expression is not true (i.e., CF_(ch) ^(1.5) does notequal 2(A^(1.5)(1/L))), then the digital value for M is incremented(blocks 765 and 770, FIG. 11a). For example, after the first evaluation,if CF_(ch) ^(1.5) is not equal to 2(A^(1.5)(1/L)) then the initial valueof M, 00001, is incremented to the value of 00010. This process repeatsuntil the expression evaluates to true. When the expression evaluates totrue, the M code has been determined, and the C bank is set based on theM code (block 755, FIG. 11a).

[0070] The process re-calculates the offset, A, after the course tuningprocedure. The variable, A, is set to an initial condition (e.g., 10000)(block 762, FIG. 11b). The circuit obtains a new sample value for the LCoscillator frequency, F_(LC), after the course tuning (block 764, FIG.11b). The frequency, F_(LC), is converted from an analog signal to adigital value using a high-speed divider. The following expression isevaluated:

CF _(LC) ² =A ²(1/L)

[0071] If the expression is not true (i.e., CF_(LC) ² does not equalA²(1/L)), and CF_(LC) ² is greater than A²(1/L), then the digital valuefor A is incremented (blocks 766, 768 and 770, FIG. 11b). If theexpression is not true (i.e., CF_(LC) ² does not equal A²(1/L)), andCF_(LC) ² is less than A²(1/L), then the digital value for A isdecremented (blocks 766, 768 and 769, FIG. 11b). This process repeatsuntil the expression evaluates to true. When the expression evaluates totrue, the new offset, A, has been calculated. The offset is stored in aregister for subsequent use (block 772, FIG. 11b).

[0072] Next, a value for the N code, and consequently L, is determined.The selection of the N code results in fine tuning a channel in the UHFspectrum. First, an initial value is set for N (block 774, FIG. 11b). Inone embodiment, N is initially set to a value of 0000. Also, the valuesfor F_(ch) and M code are selected. F_(ch) represents the frequency ofthe channel code. The M code was set from the coarse tuning stage. Theexpression, F_(ch) ^(2.0)C=A^(2.0)(1/L), is evaluated to determine if itis true (block 778, FIG. 11b). In one embodiment, these digital valuesare converted to analog current values using digital to analogconverters. The analog current values are then weighted in accordancewith the expression (e.g., A^(2.0), F_(ch) ^(2.0), etc.) using acalculator, and then compared using a comparator (See FIGS. 11 and 12).If the expression does not evaluate to true, the N code is incremented(block 780, FIG. 11b), and the expression is again evaluated with thenew N code. This process is repeated until the expression evaluates totrue. With the expression evaluates to true, the inductive bank is setbased on the N code (block 790, FIG. 11b).

[0073]FIG. 12 shows one embodiment for selecting capacitors in acapacitor bank for UHF tuning. The first column of FIG. 12 showscapacitor values for the corresponding capacitors listed in column 2.For example, capacitor C3 has a value of 32.8 pF, while capacitor C1 hasa value of 9.4 pF. Note that capacitor C0 is always selected. A binaryM-1 code for selecting capacitors in a capacitor bank is shown. Forexample, column 3 shows selection of capacitor C0 for the M-1 code of00001. The last column of FIG. 12 shows selection of capacitors from thecapacitor bank for the M-1 code of 01111. Specifically, the “01111” M-1code specifies selection of capacitors C0, C1, C2 and C3 from thecapacitor bank. The last row of FIG. 12 shows the total capacitance forthe various M-1 codes. For example, for the “00001” M-1 code, the totalcapacitance of the capacitor bank is equal to 3.7 pF. The totalcapacitance for the “01111” M-1 code is equal to 62.9 pF.

[0074]FIG. 13 is a graph that depicts the relationship between thecenter frequency of an LC bank filter and the total inductance as afunction of the N-1 code. As shown in FIG. 13, for center frequencies inthe UHF spectrum, a lower N-1 code (i.e., overall inductance) yields ahigher center frequency. FIG. 13 also depicts the relationship betweenthe N-1 code and center frequency of the LC bank filter (and the desiredchannel) for each value of the M code. For example, curve 870 shows therelationship between the center frequency of an LC bank filter and thetotal inductance as a function of the N-1 code when M is equal to 7.

[0075] As discussed above, in order to maximize the Q factor, onlycertain combinations of the N code and M code are used. FIG. 14 depictsrelationships between the selected N-1 code and center frequency forvarious combinations of the M code. Note that the relationship betweenthe center LC bank filter frequency and the N-1 code is now confined tothe center of a graph. This relationship minimizes the Q factor, andoptimizes the response of the LC filter bank.

[0076]FIGS. 15a and b show the information for selecting inductors in anL Bank during UHF tuning. The first column of FIGS. 15a and b lists thetotal inductance, which ranges from 2.85 nH to 91.2 nH, for the L bank.The second column identifies an N-1 code to obtain the correspondinginductance values of column one. For example, a decimal N−1 code of 27yields a total inductance of 3.3 nH. Column 3 of FIGS. 15a and b showsvalid selections for the N-1 code based on the frequency and the valueof the M code. For example, in column 3, a list of frequencies is shownfor valid selections of the N-1 code when the M code is equal to 1.Specifically, column 3 shows that for these frequencies and N−1 code,the M code has a valid range between 11 and 32 (decimal).

[0077]FIG. 16 illustrates various resistances for selected capacitancesof the C bank. The third column of FIG. 16 lists the resistance,R_(MOS), from the MOS transistor for the corresponding capacitor. Forexample, C2 has a R_(MOS) resistance of 0.66 ohms. The fourth columnlists the total series resistance, r_(s), from selecting thecorresponding capacitor. For example, C4 has a total series resistance,r_(s), of 1.8 ohms. The circuit selects combinations of capacitors froma C bank to reduce the total series resistance and thus maximize the Qfactor.

[0078]FIG. 17 is a timing diagram that shows timing for tuning the LCfilter bank in accordance with one embodiment. As shown in FIG. 17,there are five operations to tune the LC filter bank for a channel inthe VHF spectrum. For this embodiment, a timing signal has a frequencyof 31.25 kHz. First, a frequency measurement of the LC oscillatorfrequency is made. As shown in FIG. 17, the LC oscillator frequencymeasurement occurs in a single 16 micro second cycle. As described inFIG. 4a, an offset, A, which indicates the difference between the LCoscillator frequency and the desired frequency, is calculated. Theprocess to calculate the offset, an iterative process, occurs oversixty-four (64) steps (i.e., the value for A is determined in a loopthat consists of no more than 64 iterations). As shown in FIG. 17, eachstep occurs within 16 microseconds.

[0079] When tuning the circuit for a desired channel in the VHFspectrum, inductors for the inductor bank are selected first. In oneembodiment, the process to select the N code occurs in no more than 32steps. Again, each step occurs within a 16 micro second cycle.

[0080] After inductors for the L bank are selected, a new offset, A,based on the selected inductor bank, is calculated (see FIG. 4b). Asshown in FIG. 17, calculation of the new offset, A, occurs within nomore than 64 steps.

[0081] The fifth operation shown in FIG. 17 selects capacitors from theC. bank. As shown in FIG. 17, the process to select the M code occurs inno more than 16 steps, with each step having a period of 16 microseconds. As discussed above in conjunction with FIGS. 11a and 11 b,tuning for channels in the UHF spectrum involves selecting the M code(tuning the capacitor bank) and then selecting the N code (tuning theinductor bank).

[0082]FIG. 18 illustrates one embodiment for a functional comparatorcircuit used in one embodiment for tuning the LC filter bank. Acomparator circuit 1100 is used to evaluate expressions. In general,functional comparator circuit 1100 calculates expressions using analogcurrents. Specifically, for this embodiment, functional comparatorcircuit 1100 evaluates the expressions:

CF _(LC) ² =A ²(1/L)  (1)

CF _(Ch) ² =A ²(1/L)  (2)

CF _(ch) ^(1.5)=2(A ^(1.5)(1/L))  (3)

F _(ch) ^(1.5) C=A ^(1.5)(1/L)  (4)

[0083] The above expressions may also be written as:

C/A ²=(1/L)/F _(LC) ²  (1)

C/A ²=(1/L)/F _(ch) ²  (2)

C/2A ^(1.5)=(1/L)/F _(ch) ^(1.5)  (3)

C/A ^(1.5)=(1/L)/F _(ch) ^(1.5)  (4)

[0084] The left-hand side of the above expressions (i.e., C/A²,C/2A^(1.5), and C/A^(1.5)) are generated using transistors 1112, 1114,and 1118, switches 1120 and 1122, current sources 1130 and 1140 andcalculator 1200. Switches 1120 and 1122 are set to select either the 1.5or 2.0 exponent for the offset variable, A. For example, to evaluate theexpression C/A^(1.5), switch 1120 is set to couple the current, I_(1.5),for input to calculator 1200.

[0085] In one embodiment, current source 1130 is coupled to a digital toanalog converter to convert the digital M code value to an analogcurrent, I_(c). The analog current, I_(c), represent the capacitance andthe C bank. The current sources 1140, also coupled to a digital toanalog converter, converts the digital offset value, A, to an analogcurrent, I_(A). The output of calculator 1200, V_(out), is input tocomparator 1110. The V_(out) voltage represents a value for theleft-hand expression.

[0086] The right hand side of the above expressions (i.e., (1/L)/F_(ch)^(1.5), (1/L)/F_(LC) ², and (1/L)/F_(ch) ^(1.5)) are generated usingtransistors 1102, 1104, and 1106, switches a 1108 and 1110, and currentsources 1109 and 1111. Switches 1108 and 1110 are used to select theappropriate exponent for the frequency. For example, if the currentexpression for evaluation is (1/L)/F_(LC) ², then switch 1110 is set.The current source 1111 generates an analog current proportional to theinductor value for the L bank. In one embodiment, the current source1111 is coupled to an output from a digital to analog converter thatconverts the digital value of the N code to an analog current. Thecurrent source 1109, also coupled to a digital to analog converter,converts the frequency (LC oscillator frequency or channel codefrequency) to an analog current. The output of calculator 1200 generatesa voltage, V_(out), for the right hand expression.

[0087] The left-hand expression and right hand expression are input tocomparator 1110. The comparator 1110 compares the V_(out), generated bythe left-hand side of the expression, with the V_(out) generated by theright hand side of the expression.

[0088]FIG. 19 illustrates one embodiment for a calculator used in thefunctional comparator circuit of FIG. 18. The calculator circuitreceives, as inputs, the weighted currents for frequency or offset, aswell as the analog currents for inductance or capacitance. As shown inFIG. 19, the weighted currents for frequency or offset, A, with anexponent of two is input to transistors 1204, 1206, and 1210. Theweighted currents for frequency or offset, A, with an exponent of 1.5are input to transistors 1212 and 1222. The analog current forinductance or capacitance is input to the base of bipolar transistor1224. In turn, comparator circuit generates a voltage in accordance withthe following expression:${Vout} = {V\quad t\quad {\ln \left( {\frac{1}{Is}\frac{Inum}{Iden}} \right)}}$

[0089]FIG. 20 illustrates a plurality of frequency responses for oneembodiment of the LC filter bank. As shown in FIG. 20, the LC filterbank, through selection of different inductors and capacitors, generatesa wide range of frequency responses. The LC filter bank is a tunablefilter that may be used in circuits that operate on a wide range offrequencies, such as a television receiver.

[0090] The LC filter banks filters (i.e., discrete passive filters)enhance the performance of the tuner circuit. The use of a continuous oractive filter requires a power supply voltage (e.g., V_(cc)). The powersupply voltage exhibits a ripple due to noise on the voltage supplyline. This ripple voltage, in turn, causes unacceptable frequencyresponse characteristics on the output of the continuous amplifier.Thus, the use of the discrete or passive filters in the receiver isolatethe signal from ripple voltage, thereby improving signal quality.

[0091] Although the present invention has been described in terms ofspecific exemplary embodiments, it will be appreciated that variousmodifications and alterations might be made by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A method for tuning an inductive (“L”) andcapacitive (“C”) filter based on a desired frequency, said methodcomprising the steps of: providing at least one inductive (“L”) bankcomprising a plurality of inductors, said at least one of said inductorsin said L bank being selected for said LC filter by an N code; providingat least one capacitive (“C”) bank comprising a plurality of capacitors,said at least one of said capacitors in said C bank being selected forsaid LC filter by an M code; determining a first offset between aninitial resonant frequency of said LC filter and a desired frequency forwhich said LC filter is tuned; determining an N code, to selectivelycouple at least one inductor in said L bank, based on an initial valuefor said M code, said desired frequency, and said first offset;selecting at least one inductor in said L bank based on said N code;determining a second offset between a resonant frequency of said LCfilter and a desired frequency for which said LC filter is tuned;determining an M code, to selectively couple at least one capacitor insaid C bank, based on said value for said N code, said desiredfrequency, and said second offset; and selecting at least one capacitorin said C bank based on said M code.
 2. The method as set forth in claim1, wherein the steps of determining a first and second offset comprisesthe step of evaluating the expression: CF _(LC) ² =A ²(1/L), wherein, Arepresents said first and second offset, C represents capacitance ofsaid C bank, L represents inductance of said L bank, and F_(LC)represents said resonant frequency of said LC filter.
 3. The method asset forth in claim 2, wherein the step of evaluating the expressioncomprises the steps of: setting an initial value for said first andsecond offsets; determining whether said expression evaluates to true;setting a new value for said first and second offset if said expressiondoes not evaluate to true; and repeating the steps of determiningwhether said expression evaluates to true and setting a new value untilsaid expression evaluates to true.
 4. The method as set forth in claim3, wherein the step of determining whether said expression evaluates totrue comprises the steps of: converting said F_(LC) to a digital value;storing a digital value for said A value; storing an M code for said Cvalue; storing an N code for said L value; converting said F_(LC), A, Mcode and N code to an analog current; calculating said F_(LC) ²expression and said A expression in a calculator; generating saidCF_(LC) ² expression and said A²(1/L) expression; and comparing saidCF_(LC) ² expression with said A (1/L) expression.
 5. The method as setforth in claim 1, wherein the steps of determining an N code comprisesthe step of evaluating the expression: CF _(ch) ² =A ²(1/L), wherein, Arepresents said first offset, C represents capacitance of said C bank, Lrepresents inductance of said L bank, and F_(ch) represents said desiredfrequency.
 6. The method as set forth in claim 5, wherein the step ofevaluating the expression comprises the steps of: setting an initialvalue for inductance of said L bank; determining whether said expressionevaluates to true; setting a new value for said inductance of said Lbank if said expression does not evaluate to true; and repeating thesteps of determining whether said expression evaluates to true andsetting a new value until said expression evaluates to true.
 7. Themethod as set forth in claim 6, wherein the step of determining whethersaid expression evaluates to true comprises the steps of: convertingsaid F_(ch) to a digital value; storing a digital value for said Avalue; storing an M code for said C value; storing an N code for said Lvalue; converting said F_(ch), A, M code and N code to an analogcurrent; calculating said F_(ch) ² expression and said A² expression ina calculator; generating said CF_(ch) ² expression and said A²(1/L)expression; and comparing said CF_(ch) ² expression with said A²(1/L)expression.
 8. The method as set forth in claim 1, wherein the steps ofdetermining an N code comprises the step of evaluating the expression:CF_(ch) ^(1.5) =A ^(1.5)(1/L), wherein, A represents said second offset,C represents capacitance of said C bank, L represents inductance of saidL bank, and F_(ch) represents said desired frequency.
 9. The method asset forth in claim 8, wherein the step of evaluating the expressioncomprises the steps of: setting an initial value for capacitance of saidC bank; determining whether said expression evaluates to true; setting anew value for said capacitance of said C bank if said expression doesnot evaluate to true; and repeating the steps of determining whethersaid expression evaluates to true and setting a new value until saidexpression evaluates to true.
 10. The method as set forth in claim 9,wherein the step of determining whether said expression evaluates totrue comprises the steps of: converting said F_(ch) to a digital value;storing a digital value for said A value; storing an M code for said Cvalue; storing an N code for said L value; converting said F_(ch), A, Mcode and N code to an analog current; calculating said F_(ch) ^(1.5)expression and said A^(1.5) expression in a calculator; generating saidCF_(ch) ^(1.5) expression and said A^(1.5)(1/L) expression; andcomparing said CF_(ch) ^(1.5) expression with said A^(1.5)(1/L)expression.
 11. The method as set forth in claim 1, wherein said desiredfrequency comprises a frequency in a very high frequency (“VHF”)spectrum for television tuning.
 12. A method for tuning an inductive(“L”) and capacitive (“C”) filter based on a desired frequency, saidmethod comprising the steps of: providing at least one inductive (“L”)bank comprising a plurality of inductors, said at least one of saidinductors in said L bank being selected for said LC filter by an N code;providing at least one capacitive (“C”) bank comprising a plurality ofcapacitors, said at least one of said capacitors in said C bank beingselected for said LC filter by an M code; determining a first offsetbetween an initial resonant frequency of said LC filter and a desiredfrequency for which said LC filter is tuned; determining an M code, toselectively couple at least one capacitor in said C bank, based on aninitial value for said N code, said desired frequency, and said firstoffset; selecting at least one capacitor in said C bank based on said Mcode; determining a second offset between a resonant frequency of saidLC filter and a desired frequency for which said LC filter is tuned;determining an N code, to selectively couple at least one inductor insaid L bank, based on said value for said M code, said desiredfrequency, and said second offset; and selecting at least one inductorin said L bank based on said N code.
 13. The method as set forth inclaim 12, wherein the steps of determining a first and second offsetcomprises the step of evaluating the expression: CF _(LC) ² =A ²(1/L),wherein, A represents said first and second offset, C representscapacitance of said C bank, L represents inductance of said L bank, andF_(LC) represents said resonant frequency of said LC filter.
 14. Themethod as set forth in claim 13, wherein the step of evaluating theexpression comprises the steps of: setting an initial value for saidfirst and second offsets; determining whether said expression evaluatesto true; setting a new value for said first and second offset if saidexpression does not evaluate to true; and repeating the steps ofdetermining whether said expression evaluates to true and setting a newvalue until said expression evaluates to true.
 15. The method as setforth in claim 14, wherein the step of determining whether saidexpression evaluates to true comprises the steps of: converting saidF_(LC) to a digital value; storing a digital value for said A value;storing an M code for said C value; storing an N code for said L value;converting said F_(LC), A, M code and N code to an analog current;calculating said F_(LC) ² expression and said A² expression in acalculator; generating said CF_(LC) ² expression and said A²(1/L)expression; and comparing said CF_(LC) ² expression with said A²(1/L)expression.
 16. The method as set forth in claim 12, wherein the stepsof determining an M code comprises the step of evaluating theexpression: CF _(ch) ^(1.5)=2(A ^(1.5)(1/L)), wherein, A represents saidfirst offset, C represents capacitance of said C bank, L representsinductance of said L bank, and F_(ch) represents said desired frequency.17. The method as set forth in claim 16, wherein the step of evaluatingthe expression comprises the steps of: setting an initial value forcapacitance of said C bank; determining whether said expressionevaluates to true; setting a new value for said capacitance of said Cbank if said expression does not evaluate to true; and repeating thesteps of determining whether said expression evaluates to true andsetting a new value until said expression evaluates to true.
 18. Themethod as set forth in claim 17, wherein the step of determining whethersaid expression evaluates to true comprises the steps of: convertingsaid F_(ch) to a digital value; storing a digital value for said Avalue; storing an M code for said C value; storing an N code for said Lvalue; converting said F_(ch), A, M code and N code to an analogcurrent; calculating said F_(ch) ^(1.5) expression and said A^(1.5)expression in a calculator; generating said CF_(ch) ^(1.5) expressionand said 2(A^(1.5)(1/L)) expression; and comparing said CF_(Ch) ^(1.5)expression with said 2(A^(1.5)(1/L)) expression.
 19. The method as setforth in claim 12, wherein the steps of determining an N code comprisesthe step of evaluating the expression: CF _(Ch) ^(2.0) =A ^(2.0)(1/L),wherein, A represents said second offset, C represents capacitance ofsaid C bank, L represents inductance of said L bank, and F_(ch)represents said desired frequency.
 20. The method as set forth in claim19, wherein the step of evaluating the expression comprises the stepsof: setting an initial value for inductance of said L bank; determiningwhether said expression evaluates to true; setting a new value for saidinductance of said L bank if said expression does not evaluate to true;and repeating the steps of determining whether said expression evaluatesto true and setting a new value until said expression evaluates to true.21. The method as set forth in claim 20, wherein the step of determiningwhether said expression evaluates to true comprises the steps of:converting said F_(ch) to a digital value; storing a digital value forsaid A value; storing an M code for said C value; storing an N code forsaid L value; converting said F_(ch), A, M code and N code to an analogcurrent; calculating said F_(ch) ^(2.0) expression and said A^(2.0)expression in a calculator; generating said CF_(ch) ^(2.0) expressionand said A^(2.0)(1/L) expression; and comparing said CF_(ch) ^(2.0)expression with said A^(2.0)(1/L) expression.
 22. The method as setforth in claim 12, wherein said desired frequency comprises a frequencyin a ultra high frequency (“UHF”) spectrum for television tuning.
 23. Acircuit comprising: at least one inductive (“L”) bank comprising aplurality of inductors; at least one capacitive (“C”) bank comprising aplurality of capacitors; a plurality of switches, coupled to said L bankand said C bank, said switches for selecting at least one inductor insaid L bank based on said N code, and for selecting at least onecapacitor in said C bank based on said M code; and comparator circuitfor determining a first offset between an initial resonant frequency ofsaid LC filter and a desired frequency for which said LC filter istuned, for determining an N code, to selectively couple at least oneinductor in said L bank, based on an initial value for said M code, saiddesired frequency, and said first offset, for determining a secondoffset between a resonant frequency of said LC filter and a desiredfrequency for which said LC filter is tuned, for and determining an Mcode, to selectively couple at least one capacitor in said C bank, basedon said value for said N code, said desired frequency, and said secondoffset.